Implantable independently adjustable electrodes are used for in vivo and in vitro neurophysiological research. Prior art brain research instrumentation includes movable single channel or single electrode mechanisms which were limited to recording from single locations in the brain. Early research tended to be concentrated in sensory portions of the brain such as the visual cortex. For example, the research would seek to identify what particular stimulus in the subject's visual field would cause an individual neuron in the visual cortex to fire. The prior art single electrode mechanisms were capable of being moved to different locations in the brain but were only capable of recording from a single neuron or a small neuron cluster at a time.
The prior art also includes apparatus with multiple electrodes whose position in space is fixed relative to the other electrodes. These prior art electrodes are capable of recording timing or firing patterns of multiple neurons or multiple small clusters of neurons. The importance of being able to record timing patterns is critical to understanding higher order functions of the brain. However, the multi-channel or multi-electrode prior art devices could only be used in restrained animal subjects and were not capable of being moved within the brain. Thus, the timing patterns that could be recorded within the brain were limited by the number of electrodes and to only those patterns that occurred between the individual neurons or small neuron clusters that happen to be near the tips of the recording electrodes. Another disadvantage of the fixed array of electrodes is that the research is inherently limited to those brain functions performed by a non-moving subject.
The foregoing discussion of the prior art derives largely from U.S. Pat. No. 5,928,143 (the '143 patent) in which there is described an implantable multi-electrode microdrive array that may be made sufficiently small and light weight and it may be implanted on animals as small as rats. Referring to FIGS. 1-3, the multi-electrode microdrive array made in accordance with the '143 U.S. patent comprises an array of elongated guide cannulae referred to generally as 20 which provide a guide means for the recording electrodes. In the embodiment shown the total number of guide cannulae provided is 14. The 14 cannulae and their associated drive mechanisms are identical. Each of the guide cannulae, such as individual cannula 21 shown in FIG. 1, has an upper end 21a and a lower end 21b. The lower ends of each cannula are aligned parallel with and are adjacent each other. The 14 guide cannulae, shown in the embodiments of FIGS. 1 and 2, easily fit into a passageway 8 formed in the skull 9 of the subject animal.
The upper end 21a of cannula 21 is inclined outwardly from the central vertical axis A of the apparatus 10 by preferably an angle of 30°. Other angles may be used. By inclining the upper ends of the array of cannulae 20 outwardly, as shown best in FIG. 2, sufficient spacing is obtained between adjacent cannulae that electrodes carried within the cannulae are capable of being independently adjusted relative to one another.
A support means 40 is provided for the array of guide cannulae 20. The support means is preferably a mechanical plastic core. The plastic core has an upper end 42 and a lower end 43. A first passageway 44 is formed in the lower end 43 of the plastic core, and is adapted to receive a cylindrical bushing 49 with an internal passageway 48 through which the lower ends of the array of guide cannulae 20 extend, as shown in FIG. 3. The support means or plastic core 40 also has a plurality of inclined passageways such as 46 formed in its upper portion 42 for each upper portion of each guide cannula. Each of the inclined passageways such as 46 communicates with the passageway 48 formed by bushing 49 and extends upwardly and preferably outwardly at an angle of 30° of the vertical axis A. Each of the guide cannulae remains fixed relative to support means 40 with upper end 21a terminating just below the upper surface 42a of support means 40.
Each of said guide cannulae in the array 20 carries one or more electrodes. For example, guide cannula 21 shown in FIG. 1 carries in the preferred embodiment a group of four electrodes 51, which is referred herein as a tetrode. The tetrode 51 has an upper end 51a which is located in the upper portion 21a of guide cannula 21. The tetrode 51 has a lower portion 51b which is capable of being moved downwardly into the brain 7 to various depths.
According to the '143 patent, the preferred electrode assembly includes a tetrode which comprises four recording probes made by twisting together four strands of polyamide-coated, 14 micron diameter, nichrome wires. To obtain adherence and stiffness, the insulation is briefly softened by heating, while the wires are under tension, and then allowed to cool. Wires are cut flat at the same level and each tip is gold-plated separately to reduce the impedance of individual electrodes to 400-500 K-ohm. The overall diameter of each tetrode is approximately 40 microns. The tetrode is mounted and glued into two nested polyamide tubes, those tubes being 78 and 110 microns outside diameter, respectively, which are then mounted in the support means 40. The smaller tubing (78 micron O.D.) forms each guide cannula 21 and the larger tubing (110 micron O.D.) forms each drive cannula 71.
Electrode adjustment means shown generally as 70 (FIG. 1) are connected to the upper end of each electrode and each guide cannula. The electrode adjustment drive means 70 is capable of moving the electrode or electrode bundle in each guide cannula independently of the electrodes carried in the other guide cannulae in the array 20. As shown in FIG. 1, each electrode adjustment means 70 is capable of moving between the position shown in solid lines downwardly to the position shown in phantom as 70a. Electrode adjustment means 70 in the preferred embodiment shown in FIG. 1 includes a drive cannula 71 slidably mounted over the upper end 21a of guide cannula 21, electrode drive means 75, adjustment rod 72 and guide rod 85 described below. Each guide cannula 21 (typically 78 micron O.D.) slidably nests in drive cannula 71 (typically 110 micron O.D.). The top of electrode 51a slides through and is attached to the top of drive cannula 71 by glue. A plurality of adjustment rod means 72 (FIG. 3) are provided, each of which is carried by support means 40 and each of which is mounted parallel to its respective guide cannula 21. Adjustment rod means 72 is preferably an 0.080″ headless stainless screw, threaded into support means 40.
Referring also to FIGS. 4a and 4b, an electrode drive means 75 is carried by each adjustment rod 72 as shown best in FIG. 3. Electrode drive means 75 includes a cylindrically shaped nut 76 with a threaded inner bore 77 which threads onto threaded rod 72. Nut 76 has a turning slot 78 formed in its upper surface.
Nut 76 is connected to a generally triangular shaped bridge 80. Bridge 80 has a first bore 81 formed therein for carrying drive cannula 71. Nut 76 has an upper flange 76a and a lower flange 76b which cause bridge 80 to move upwardly and downwardly on rod 72 as the nut 76 is turned. Bridge 80 has a second bore 82 formed therein for receiving a guide rod 85 (FIG. 2). Guide rod 85 prevents rotation of bridge 80 relative to adjustment rod 72 as nut 76 is rotated. Guide rod 85 is parallel to adjustment rod 72 and is cemented into support means 40.
Referring also to FIGS. 5a-5c, an electrode adjustment tool 100 is provided having a hollow drive sleeve 101 adapted to slide over the top of adjustment rod 72 (FIG. 3). A drive tip 102 is formed at the distal end 103 of sleeve 101. Drive tip 102 engages turning slot 78. A turning knob 105 is carried near the proximal end 104 of sleeve 101 and carries a scale 108 to indicate the motion imparted to an electrode. Rotation of knob 105 imparts axial motion to drive cannula 71 and to the electrode or electrodes 51 carried within that drive cannula.
While the multi-electrode microdrive array described in the aforesaid '143 patent provides significant improvements over prior art devices in providing an increased number of independently adjustable recording electrodes in an array that is small enough and light weight enough to be easily carried on the skull of the subject animal, while providing free motion of the subject, the multi-electrode microdrive array of the '143 patent is expensive to manufacture requiring painstaking and time consuming machining and assembly. The multi-electrode microdrive array of the '143 patent also is difficult to use in the field requiring painstaking and time consuming manipulation to adjust individual electrodes into position.